1. Field of the Invention
The present invention relates to a rewritable semiconductor memory such as a flash memory, and more particularly to a technology for writing an analog value to a memory cell transistor of a semiconductor memory.
2. Description of Related Art
As a conventional memory, an analog storage flash memory, for example, is known. An analog storage flash memory is a flash memory where an analog value can be written to a memory cell by the analog controlling of charge amount stored in a floating gate. An analog storage flash memory is used for the internal memory of a LSI (Large Scale Integrated Circuit) for reproducing a recorded sound.
The downside of an analog storage flash memory is that a sufficient writing accuracy cannot be obtained when the dispersion of writing characteristics of the memory cell transistor is high. In a memory cell transistor, the writing speed disperses due to manufacturing dispersion and for other reasons, so if the writing speed of the memory cell transistor is very fast, the rise of the ON/OFF threshold value per one writing becomes very high, and the actual ON/OFF threshold value obtained by writing may exceed a desired setup value considerably.
It is an object of the present invention to provide an analog storage semiconductor memory which writing accuracy is sufficiently high.
In this, in accordance with the first standpoint of the present invention, an analog storage semiconductor memory according to the present invention includes a memory cell driver for storing an analog data to a memory cell transistor by setting an operation threshold voltage of the memory cell transistor based on the voltage corresponding to the analog data, wherein the setting is executed by supplying an analog voltage with decreasing voltage as the operation threshold voltage approaches the voltage corresponding to the analog data, to a control gate of the memory cell transistor.
Moreover, in accordance with the second standpoint of the present invention, an analog storage semiconductor memory according to the present invention includes a write voltage supply circuit for supplying a plurality of types of write voltage to a control gate of a memory cell transistor; a read voltage supply circuit for supplying a plurality of types of read voltage to the control gate of the memory cell transistor; and a write control circuit for executing an operation of making the read voltage supply circuit supply the read voltage and judging a threshold voltage of the memory cell transistor, and for executing an operation of making the write voltage supply circuit supply the write voltage with decreasing voltage as the operation threshold voltage approaches the voltage corresponding to the analog data.
In addition, in accordance with the third standpoint of the present invention, an analog storage semiconductor memory according to the present invention comprises a write voltage supply circuit for supplying n types (nxe2x89xa72) of write voltage Vw1, . . . , Vwn (Vw1 greater than  . . .  greater than Vwn) to a control gate of a memory cell transistor; a read voltage supply circuit for supplying n types of read voltage Vr1, . . . , Vrn (Vr1 less than  . . .  less than Vrn) to the control gate of the memory cell transistor; and a write control circuit for sequentially executing, from m=1 to m=n, the m-th write cycle where the write operation at the m-th write voltage Vwm and read operation at the m-th read voltage Vrm are repeated until the operation threshold value of the memory cell transistor exceeds the m-th read voltage Vrm.
According to these configurations, an analog value can be written by decreasing the write voltage as an operation threshold value approaching the voltage Vrn. As a result, the write accuracy of a semiconductor memory can be sufficiently high.